Flat Tube, Platelike Body for Making the Flat Tube and Heat Exchanger

ABSTRACT

A flat tube includes a pair of upper and lower walls opposed to each other, two side walls interconnecting the upper and lower walls at left and right opposite side edges thereof, and a covering wall provided over the outer surface of the left side wall and brazed to the left side wall. The left side wall is made from a side wall forming portion projecting downward from the upper wall integrally therewith and a side wall forming portion projecting upward from the lower wall integrally therewith by brazing these portions as butted against each other. The covering wall is made by folding a covering wall forming portion formed by extending one side edge of the upper wall. The left side wall including the wall forming portion of the upper wall and the wall forming portion of the lower wall can be protected.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a) claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date of Provisional Application No. 60/601,129 filed Aug. 13, 2004 pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to flat tubes for use as heat exchange tubes for heat exchangers, such as refrigerant tubes in condensers or evaporators for motor vehicle air conditioners, oil tubes for motor vehicle oil coolers, water tubes for motor vehicle radiators and heat medium tubes for heater cores, platelike bodies for making such flat tubes, and heat exchangers comprising such flat tubes.

The term “aluminum” as used herein and in the appended claims includes aluminum alloys in addition to pure aluminum.

BACKGROUND ART

In recent years, widely used, for example, in motor vehicle air conditioners in place of conventional serpentine condensers are condensers which comprise, as shown in FIG. 7, a pair of headers 30, 31 arranged in parallel as spaced apart from each other, flat refrigerant tubes 32 aluminum arranged in parallel at a spacing between the two headers 30, 31 and each joined at its opposite ends to the two headers 30, 31, corrugated aluminum fins 33 each disposed in an air flow clearance between each adjacent pair of flat tubes 32 and brazed to the adjacent tubes 32, an inlet pipe 34 connected to the upper end of peripheral wall of the first 30 of the headers, an outlet pipe 35 connected to the lower end of peripheral wall of the second 31 of the headers, a first partition 36 provided inside the first header 30 and positioned above the midportion thereof, and a second partition 37 provided inside the second header 31 and positioned below the midportion thereof. The number of flat tubes 32 positioned above the first partition 36, the number of flat tubes 32 arranged between the first partition 36 and the second partition 37 and the number of flat tubes 32 positioned below the second partition 37 decrease from above downward to provide groups of channels. A refrigerant flowing into the inlet pipe 34 in a vapor phase flows zigzag through the units of channel groups in the condenser before flowing out from the outlet pipe 35 in a liquid phase. The condensers of the construction described are called multiflow condensers, and realize high efficiencies, lower pressure losses and supercompactness.

It is required that the flat refrigerant tube 32 of the condenser described be excellent in heat exchange efficiency and have pressure resistance since the gaseous refrigerant of high pressure is introduced thereinto. Moreover, the tube needs to be small in wall thickness and low in height so as to make the condenser compact.

For example, the publication of JP-A No. 6-281373 discloses a known flat tube which is excellent in heat exchange efficiency for use in the condenser described above. The flat tube disclosed in the publication comprises a pair of flat walls opposed to each other, opposite side walls interconnecting respective opposed pairs of side edges of the flat walls, and a plurality of reinforcing walls interconnecting the pair of flat walls, extending longitudinally of the tube and spaced from one another by a predetermined distance, the flat tube having a plurality of parallel fluid channels in its interior.

Such a flat tube is fabricated from a platelike body in the form of a single metal plate in its entirety and having two flat wall forming portions of same width for making the two flat walls, a connecting portion interconnecting the flat wall forming portions for making one of the side walls, a side wall forming portion formed on each flat wall forming portion integrally therewith and projecting from one side edge thereof opposite to the connecting portion for making the other side wall, and a plurality of reinforcing wall forming portions projecting from each flat wall forming portion integrally therewith, by bending the platelike body into the shape of a hairpin at the connecting portion, brazing the side wall forming portions as butted against each other and brazing each reinforcing wall forming portion on one of the flat wall forming portions to the corresponding reinforcing wall forming portion on the other wall forming portion as butted thereagainst. The side wall made from the two side wall forming portions by brazing these portions as butted against each other has a thickness smaller than the thickness of the side wall provided by the connecting portion.

When the flat tube disclosed in the above publication is used in the condenser described and if a flying stone or the like collides with the side wall made from the two side wall forming portions, the side wall is likely to break due to the resulting impact.

An object of the present invention is to overcome the above problem and to provide a flat tube wherein at least one of opposite side walls thereof is protected and a platelike body for making the flat tube.

DISCLOSURE OF THE INVENTION

To fulfill the above object, the present invention comprises the following modes.

1) A flat tube comprising a pair of flat walls opposed to each other and two side walls interconnecting the flat walls at opposite side edges thereof, at least one of the opposite side edges of one of the flat walls being provided with a covering wall integral therewith and covering an outer surface of at least one of the side walls.

2) A flat tube according to par. 1) wherein the covering wall is made by folding a covering wall forming portion formed by extending said at least one side edge of one of the flat walls.

3) A flat tube according to par. 1) wherein the covering wall is joined to the outer surface of said at least one side wall.

4) A flat tube according to par. 1) wherein said at least one side wall has a smaller thickness than the other side wall, and the covering wall is provided over the outer surface of the side wall of smaller thickness.

5) A flat tube according to par. 4) wherein the side wall of smaller thickness is made from side wall forming portions projecting from the corresponding side edges of the two flat walls by joining the side wall forming portions as butted against each other.

6) A flat tube according to par. 5) wherein the covering wall covers at least a joint between butted ends of the side wall forming portions in the entire outer surface of the side wall of smaller thickness.

7) A flat tube according to par. 5) wherein the covering wall covers the entire outer surface of the side wall of smaller thickness.

8) A flat tube according to par. 4) wherein the side wall of large thickness is integral with the two flat walls.

9) A flat tube according to par. 1) wherein a plurality of reinforcing walls extending longitudinally of the tube are arranged at a spacing between the two flat walls and interconnect the two flat walls.

10) A flat tube according to par. 9) wherein the reinforcing walls are made from reinforcing wall forming portions projecting from each of the two flat walls integrally therewith by joining each corresponding pair of reinforcing wall forming portions of the flat walls as butted against each other.

11) A platelike body for making a flat tube comprising a pair of flat walls opposed to each other and two side walls interconnecting the flat walls at opposite side edges thereof, at least one of the opposite side edges of one of the flat walls being provided with a covering wall integral therewith and covering an outer surface of one of the side walls,

the platelike body being in the form of a single metal plate in its entirety and comprising two flat wall forming portions, a connecting portion interconnecting the two flat wall forming portions integrally therewith for making one of the side walls, two side wall forming portions projecting from the respective flat wall forming portions and each formed at a side edge of the flat wall forming portion opposite to the connecting portion, and a covering wall forming portion provided by extending one of the flat wall forming portions at the side edge thereof opposite to the connecting portion.

12) A flat tube making platelike body according to par. 11) wherein the side wall forming portions have a thickness smaller than the height of the connecting portion.

13) A flat tube making platelike body according to par. 11) wherein the covering wall forming portion has a width not greater than the combined height of the two side wall forming portions and larger than the height of the side wall forming portion of the flat wall forming portion provided with the covering wall forming portion.

14) A flat tube making platelike body according to par. 11) wherein the covering wall forming portion has a width equal to the combined height of the two side wall forming portions.

15) A flat tube making platelike body according to par. 11) wherein the metal plate is made by rolling an aluminum brazing sheet having a brazing material layer over opposite surfaces thereof, the platelike body being covered with the brazing material layer in its entirety except at an outer side surface of the side wall forming portion of the flat wall forming portion not provided with the covering wall forming portion and an outer end face of the covering wall forming portion.

16) A process for fabricating a flat tube characterized by folding a platelike body according to par. 11) into a hairpin form at opposite sides of the connecting portion to butt the side wall forming portions against each other, folding the covering wall forming portion to cover outer surfaces of the side wall forming portions therewith, and brazing outer ends of the side wall forming portions to each other and the covering wall forming portion to the outer surfaces of the side wall forming portions at the same time.

17) A heat exchanger comprising a pair of headers arranged in parallel as spaced apart from each other, a plurality of flat tubes each according to any one of pars. 1) to 10), the flat tubes being arranged in parallel at a spacing between the headers and having opposite ends joined to the respective headers, and fins arranged in respective air passing clearances between respective adjacent pairs of flat tubes and each brazed to the pair of flat tubes adjacent thereto.

18) A refrigeration cycle comprising a compressor, a condenser and an evaporator, the condenser comprising a heat exchanger according to par. 17).

19) A vehicle having installed therein a refrigeration cycle according to par. 18) as a motor vehicle air conditioner.

20) A refrigeration cycle comprising a compressor, a condenser and an evaporator, the evaporator comprising a heat exchanger according to par. 17).

21) A vehicle having installed therein a refrigeration cycle according to par. 20) as a motor vehicle air conditioner.

22) A process for fabricating a heat exchanger characterized by making a plurality of folded bodies from flat tube making platelike bodies each according to par. 11) by folding each of the platelike bodies into a hairpin form at opposite sides of the connecting portion to butt the side wall forming portions against each other and thereafter folding the covering wall forming portion to cover outer surfaces of the two side wall forming portions therewith, preparing a pair of headers each having folded body inserting holes arranged at a spacing and fins, arranging the pair of headers as spaced apart from each other and arranging the folded bodies and the fins alternately, inserting opposite ends of the folded bodies into the respective inserting holes of the headers, fabricating flat tubes by brazing the side wall forming portions of each folded body to each other and the covering wall forming portion thereof to the outer surfaces of the side wall forming portions, and brazing the flat tubes to the headers and each fin to the flat tubes adjacent thereto simultaneously with the fabrication of the flat tubes.

With the flat tube according to par. 1), the side wall can be protected with the covering wall. When the flat tube is used for a condenser or like heat exchanger, therefore, the side wall can be prevented from becoming damaged or broken owing to the impact of a collision with an extraneous matter, whereby the refrigerant or the like can be prevented from leaking.

With the flat tube according to par. 2), the covering wall can be made relatively easily.

With the flat tube according to par. 3), the side wall can be protected with the covering wall effectively.

In the case of the flat tube according to par. 4), the side wall of smaller thickness has a lower strength than the side wall of great thickness to entail the likelihood that the problem described above will appear markedly, whereas even in this case, the side wall of smaller thickness can be protected with the covering wall provided over this wall.

With the flat tube according to par. 5), the side wall comprising side wall forming portions joined as butted against each other is likely to be insufficient in strength, whereas the side wall of smaller thickness can be protected with the covering wall provided over this wall.

The flat tube according to par. 6) is likely to be insufficient in the strength of the joint of the side wall of smaller thickness, but the joint can be protected with the covering wall. Further in the case where the side wall forming portions are brazed as butted against each other, the inside-to-outside length of the brazed joint as observed in cross section is short, so that corrosion is liable to develop through the portion of brazing material present between the butted ends of the side wall forming portions to entail lower corrosion resistance. However, when the joint is covered with the covering wall, which is further joined to the side wall as by brazing, the inside-to-outside length of the brazed joint becomes elongated in cross section to result in improved corrosion resistance.

With the flat tube according to par. 7), the side wall of smaller thickness can be protected with the covering wall with improved effectiveness.

The flat tube according to any one of pars. 1) to 8) can be fabricated relatively easily by folding a platelike body according to par. 11) into a hairpin form at opposite sides of the connecting portion to butt the side wall forming portions against each other, thereafter folding the covering wall forming portion to cover the outer surfaces of the side wall forming portions therewith, and subsequently brazing the ends of the side wall forming portions to each other and the covering wall forming portion to the outer surfaces of the side wall forming portions.

The flat tube described in par. 6) can be made from the platelike body according to par. 13).

The flat tube described in par. 7) can be made from the platelike body according to par. 14).

When the platelike body according to par. 15) is used, the side wall forming portions can be joined as butted against each other, and the covering wall can be joined to the outer surfaces of the sidewall forming portions, utilizing the brazing material layer of the aluminum brazing sheet.

The flat tube according to any one of pars. 1) to 8) can be made relatively easily by the tube fabricating process according to par. 16).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a flat tube of the invention.

FIG. 2 includes diagrams showing some steps of a process for making the flat tube from a platelike body.

FIG. 3 is a fragmentary sectional view showing FIG. 2( e) on an enlarged scale.

FIG. 4 is a cross sectional view showing a modified embodiment of the flat tube.

FIG. 5 is a front view showing a platelike body making the flat tube of FIG. 4.

FIG. 6 is a view corresponding to FIG. 3 and showing a step of a process for making the flat tube from the platelike body of FIG. 5.

FIG. 7 is a perspective view showing a condenser for use in motor vehicle air conditioners.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below with reference to the drawings. In the following description, the upper and lower sides, and the left- and right-hand sides of FIG. 1 and FIG. 2( a) will be referred to as “upper,” “lower,” “left” and “right,” respectively.

FIG. 1 shows a flat tube, FIG. 2 shows some steps of a process for making the flat tube from a platelike body, and FIG. 3 is a fragmentary view of FIG. 2( e) on an enlarged scale.

With reference to FIG. 1, the flat tube 1 is made of aluminum and comprises an upper and a lower flat wall 2, 3 (a pair of flat walls) opposed to each other, left and right opposite side walls 4, 5 interconnecting the upper and lower walls 2, 3 at left and right opposite side edges thereof, a covering wall 6 integral with the left side edge of the upper wall 2 and provided over the entire outer surface of the left side wall 4, and a plurality of reinforcing walls 7 interconnecting the upper and lower walls 2, 3, extending longitudinally of the tube and spaced from one another by a predetermined distance as positioned between the left and right side walls 4, 5, the tube 1 having parallel fluid channels 8 formed inside thereof. Although not shown, each of the reinforcing walls 7 is provided with communication holes for causing each adjacent pair of fluid channels 7 to communicate with each other therethrough, the communication holes in the entire tube being in a staggered arrangement when seen from above.

The left side wall 4 comprises a side wall forming portion 9 projecting downward from the left side edge of the upper wall 2 integrally therewith and a side wall forming portion 11 projecting upward from the left side edge of the lower wall 3 integrally therewith, and is formed by causing these portions 9, 11 to butt against each other and brazing these portions in this state. The right side wall 5 is made integral with the upper and lower walls 2, 3. The left side wall 4 has a smaller thickness than the right side wall 5.

The covering wall 6 is made from a covering wall forming portion 12 (see the chain lines in FIG. 1) which is made by extending the left side edge of the upper wall 2 leftward, by bending or folding the portion 12 to extend along the outer surface of the left side wall 4. The covering wall 6 is brazed to the left side wall 4, i.e., to the enter outer surface of the assembly of the two side wall forming portions 9, 11.

The reinforcing walls 7 comprise reinforcing wall forming portions 13 projecting downward from the upper wall 2 integrally therewith and reinforcing wall forming portions 14 projecting upward from the lower wall 3 integrally therewith, and are formed by butting the wall forming portions 13 to against the respective corresponding wall forming portions 14 and brazing the respective corresponding pairs of portions 13, 14.

The flat tube 1 is fabricated using a platelike body 15 in the manner shown in FIG. 2.

The platelike body 15 is made from an aluminum brazing sheet having a brazing material layer over opposite surfaces thereof in its entirety by rolling. As shown in FIG. 2( a), the platelike body 15 comprises an upper wall forming portion 16 (flat wall forming portion) and a lower wall forming portion 17 (flat wall forming portion) which are flat and have the same width for making the upper and lower walls 2, 3, a connecting portion 18 interconnecting the upper and lower wall forming portions 16, 17 integrally therewith for making the right side wall 5, side wall forming portions 9, 11 formed respectively on the upper wall forming portion 16 and the lower wall forming portion 17 integrally therewith and each projecting upward from one side edge thereof opposite to the connecting portion 18 for making the left side wall 4, a covering wall forming portion 12 formed by extending the upper wall forming portion 16 outward with respect to the left-right direction (i.e., rightward) from the side edge (right side edge) thereof opposite to the connecting portion 18, and reinforcing wall forming portions 13, 14 projecting upward respectively from the upper wall forming portion 16 and the lower wall forming portion 17 integrally therewith and arranged at a predetermined spacing in the left-right direction. The reinforcing wall forming portions 13 on the upper wall forming portion 16 and the reinforcing wall forming portions 14 on the lower wall forming portion 17 are symmetrical about the center line of the platelike body 15 with respect to the widthwise direction thereof.

The connecting portion 18 has a vertical height larger than the thickness of the upper and lower wall forming portions 16, 17 and the thickness of the side wall forming portions 9, 11. The side wall forming portions 9, 11 have a larger thickness than the reinforcing wall forming portions 13, 14. The side wall forming portions 9, 11 have a height of projection which is approximately half of the left-to-right width of the connecting portion 18.

The covering wall forming portion 12 has a left-to-right width equal to the combined height of projection of the two side wall forming portions 9, 11, and a thickness equal to the thickness of the upper wall forming portion 16.

By rolling an aluminum brazing sheet clad with a brazing material layer over opposite surfaces thereof into the platelike body 15, the upper wall forming portion 16, lower wall forming portion 17, connecting portion 18, side wall forming portions 9, 11, covering wall forming portion 12 and reinforcing wall forming portions 13, 14 are made integral, whereby the platelike body 15 is covered with the brazing material layer in its entirety except at the outer side surface (left side surface) of the side wall forming portion 11 on the lower wall forming portion 17 and the outer end face (right end face) of the covering wall forming portion 12. In other words, a brazing material layer 19 is formed over the upper and lower surfaces of the upper wall forming portion 16 and the lower wall forming portion 17, over the upper and lower surfaces of the connecting portion 18, over the left and right side surfaces of the part of the connecting portion 18 which projects upward beyond the upper and lower wall forming portions 16, 17, over the end faces and left and right side surfaces of the side wall forming portion 9 and the reinforcing wall forming portions 13, 14, over the end face and right side surfaces of the side wall forming portion 11 and over the upper and lower surfaces of the covering wall forming portion 12, while no brazing material layer is formed over the left side surface of the side wall forming portion 11 on the lower wall forming portion 17 and the outer end face of the covering wall forming portion 12 (see FIG. 3).

The platelike body 15 for making the flat tube is progressively bent at opposite sides of the connecting portion 18 [see FIG. 2( b)], and is finally folded into a hairpin form to butt the side wall forming portions 9, 11, as well as each corresponding pair of reinforcing wall forming portions 13, 14, against each other [see FIG. 2( c)]. At this time, the connecting portion 18 forms the right side wall 5.

The covering wall forming portion 12 is then folded (see [FIG. 2( d)] to cause the portion 12 to extend along the outer surfaces of the two side wall forming portions 9, 11 and to obtain a folded body 20 [see FIG. 2( e)]

The folded body 20 is thereafter heated to a predetermined temperature to braze the side wall forming portions 9, 11, as well as each corresponding pair of first and second reinforcing wall forming portions 13, 14, to each other utilizing the above-mentioned brazing material layer and to thereby form the left side wall 4 and the reinforcing walls 7. The upper wall 2 is further formed by the upper wall forming portion 16 and the lower wall 3 by the lower wall forming portion 17. The covering wall forming portion 12 is brazed to the left side wall 4 with the brazing material layer to thereby form the covering wall 6. In this way, the flat tube 1 is fabricated.

When the flat tube 1 is to be used, for example, as the refrigerant tube of the condenser shown in FIG. 7, such flat tubes 1 may be produced simultaneously with the fabrication of the condenser. More specifically, the condenser is fabricated in the following manner. First prepared are a plurality of folded bodies 20, a pair of aluminum headers 30, 31 each having folded body inserting holes, and a plurality of corrugated aluminum fins 33. The pair of headers 30, 31 are then arranged as spaced apart, the folded bodies 20 and the fins 33 are arranged alternately, and opposite ends of the folded bodies 20 are inserted into the respective inserting holes of the headers 30, 31. The resulting assembly is thereafter heated at a predetermined temperature to braze the two sidewall forming portions 9, 11, as well as each corresponding pair of reinforcing wall forming portions 13, 14, of each folded body 20 to each other and braze the covering wall forming portion 1 to the side wall forming portions 9, 11 thereof, whereby flat tubes 1 are made. At the same time, the flat tubes 1 are brazed to the headers 30, 31, and each of the corrugated fins 33 is brazed to the flat tubes 1 adjacent thereto. The brazing material layers of the flat tube making platelike bodies 15 are used for brazing. In this way, the condenser is fabricated.

FIG. 4 shows a modified embodiment of the flat tube. FIG. 5 shows a platelike body making the flat tube of FIG. 4. FIG. 6 shows a step of a process for making the flat tube from a platelike body on an enlarged scale.

In the case of a flat tube 21 shown in FIG. 4, a left side edge of the lower surface of the lower wall 3 serves as a slope 22 slanting diagonally upward toward the left. The lower end of the covering wall 6 is folded rightward so as to be engaged with the slope 22, and the covering wall 6 is brazed to the outer surface of the left side wall 4 and the lower wall 3. Other structural features of the flat tube 21 are similar to those of the flat tube 1 shown in FIG. 1.

In the case of a platelike body 25 used for making the flat tube 21, a lower wall forming portion 17 is provided at a left side edge of the lower surface thereof with a slope 22 slanting diagonally upward toward the left. The end portion of the covering wall forming portion 12 is tapered, and a right side edge of the lower surface of the covering wall forming portion 12 serves as a slope 26 slanting diagonally upward toward the right.

By rolling an aluminum brazing sheet clad with a brazing material layer over opposite surfaces thereof into the platelike body 25, the upper wall forming portion 16, lower wall forming portion 17 having the slope 22, connecting portion 18, side wall forming portions 9, 11, covering wall forming portion 12 wherein the end portion thereof is tapered, and reinforcing wall forming portions 13, 14 are made integral, whereby the platelike body 25 is covered with the brazing material layer in its entirety except at the outer side surface (left side surface) of the side wall forming portion 11 on the lower wall forming portion 17. In other words, unlike in the platelike body 15 shown in FIG. 2, in the platelike body 25 the covering wall forming portion 12 is covered with the brazing material layer in its entirety including the slope 26.

Other structural features of the platelike body 25 are similar to those of the platelike body 15 shown in FIG. 2 (a).

The flat tube 21 is manufactured from the platelike body 25 in a manner similar to that for the flat tube 1 except in that the covering wall forming portion 12 is folded to cause the portion 12 to extend along the outer surfaces of the two side wall forming portions 9, 11, and then the end portion of the covering wall forming portion 12 is folded so as to be engaged with the slope 22 of the lower wall forming portion 17.

The heat exchanger comprising flat tubes 1, 21 described is used in vehicles, e.g., motor vehicles, equipped with a refrigeration cycle wherein a chlorofluorocarbon refrigerant is used and which serves as a motor vehicle air conditioner and comprises a compressor, condenser and evaporator, in order to serve as the condenser of the cycle, or as the evaporator thereof. The heat exchanger may further be installed in motor vehicles as an oil cooper or radiator comprising the flat tube 1 described above.

Further the flat tube 1, 21 described may be used for gas coolers or evaporators in vehicles, e.g., motor vehicles, equipped with a supercritical refrigeration cycle wherein CO₂ refrigerant or like supercritical refrigerant is used and which serves as an air conditioner and comprises a compressor, gas cooler, evaporator, pressure reducer and an intermediate heat exchanger for subjecting to heat exchange the refrigerant flowing out of the gas cooler and the refrigerant flowing out of the evaporator.

Although the flat tube is made from a platelike body comprising a single aluminum brazing sheet according to the foregoing embodiment, the material is not limited to the body; the invention also includes a flat tube which is made from an extrudate comprising a pair of flat walls, opposite side walls integral with the flat walls, reinforcing walls integral with the flat walls and a covering wall forming portion in the form of an extension of one of the flat walls and which is made by folding the covering wall forming portion to cause this portion to extend along the outer surface of one of the side walls.

INDUSTRIAL APPLICABILITY

The present invention provides a flat tube which is suitable for use as heat exchange tubes for heat exchangers, such as refrigerant tubes in condensers or evaporators for motor vehicle air conditioners, oil tubes for motor vehicle oil coolers, water tubes for motor vehicle radiators and heat medium tubes for heater cores. 

1: A flat tube comprising a pair of flat walls opposed to each other and two side walls interconnecting the flat walls at opposite side edges thereof, at least one of the opposite side edges of one of the flat walls being provided with a covering wall integral therewith and covering an outer surface of at least one of the side walls. 2: A flat tube according to claim 1 wherein the covering wall is made by folding a covering wall forming portion formed by extending said at least one side edge of one of the flat walls. 3: A flat tube according to claim 1 wherein the covering wall is joined to the outer surface of said at least one side wall. 4: A flat tube according to claim 1 wherein said at least one side wall has a smaller thickness than the other side wall and the covering wall is provided over the outer surface of the side wall of smaller thickness. 5: A flat tube according to claim 4 wherein the side wall of smaller thickness is made from side wall forming portions projecting from the corresponding side edges of the two flat walls by joining the side wall forming portions as butted against each other. 6: A flat tube according to claim 5 wherein the covering wall covers at least a joint between butted ends of the side wall forming portions in the entire outer surface of the side wall of smaller thickness. 7: A flat tube according to claim 5 wherein the covering wall covers the entire outer surface of the side wall of smaller thickness. 8: A flat tube according to claim 4 wherein the side wall of large thickness is integral with the two flat walls. 9: A flat tube according to claim 1 wherein a plurality of reinforcing walls extending longitudinally of the tube are arranged at a spacing between the two flat walls and interconnect the two flat walls. 10: A flat tube according to claim 9 wherein the reinforcing walls are made from reinforcing wall forming portions projecting from each of the two flat walls integrally therewith by joining each corresponding pair of reinforcing wall forming portions of the flat walls as butted against each other.
 11. A platelike body for making a flat tube comprising a pair of flat walls opposed to each other and two side walls interconnecting the flat walls at opposite side edges thereof, at least one of the opposite side edges of one of the flat walls being provided with a covering wall integral therewith and covering an outer surface of one of the side walls, the platelike body being in the form of a single metal plate in its entirety and comprising two flat wall forming portions a connecting portion interconnecting the two flat wall forming portions integrally therewith for making one of the side walls, two side wall forming portions projecting from the respective flat wall forming portions and each formed at a side edge of the flat wall forming portion opposite to the connecting portion, and a covering wall forming portion provided by extending one of the flat wall forming portions at the side edge thereof opposite to the connecting portion. 12: A flat tube making platelike body according to claim 11 wherein the side wall forming portions have a thickness smaller than the height of the connecting portion. 13: A flat tube making platelike body according to claim 11 wherein the covering wall forming portion has a width not greater than the combined height of the two side wall forming portions and larger than the height of the side wall forming portion of the flat wall forming portion provided with the covering wall forming portion. 14: A flat tube making platelike body according to claim 11 wherein the covering wall forming portion has a width equal to the combined height of the two side wall forming portions. 15: A flat tube making platelike body according to claim 11 wherein the metal plate is made by rolling an aluminum brazing sheet having a brazing material layer over opposite surfaces thereof the platelike body being covered with the brazing material layer in its entirety except at an outer side surface of the side wall forming portion of the flat wall forming portion not provided with the covering wall forming portion and an outer end face of the covering wall forming portion. 16: A process for fabricating a flat tube characterized by folding a platelike body according to claim 11 into a hairpin form at opposite sides of the connecting portion to butt the side wall forming portions against each other, folding the covering wall forming portion to cover outer surfaces of the side wall forming portions therewith, and brazing ends of the side wall forming portions to each other and the covering wall forming portion to the outer surfaces of the side wall forming portions at the same time. 17: A heat exchanger comprising a pair of headers arranged in parallel as spaced apart from each other, a plurality of flat tubes each according to claim 1, the flat tubes being arranged in parallel at a spacing between the headers and having opposite ends joined to the respective headers, and fins arranged in respective air passing clearances between respective adjacent pairs of flat tubes and each brazed to the pair of flat tubes adjacent thereto. 18: A refrigeration cycle comprising a compressor, a condenser and an evaporator, the condenser comprising a heat exchanger according to claim
 17. 19: A vehicle having installed therein a refrigeration cycle according to claim 18 as a motor vehicle air conditioner. 20: A refrigeration cycle comprising a compressor, a condenser and an evaporator, the evaporator comprising a heat exchanger according to claim
 17. 21: A vehicle having installed therein a refrigeration cycle according to claim 20 as a motor vehicle air conditioner. 22: A process for fabricating a heat exchanger characterized by making a plurality of folded bodies from flat tube making platelike bodies each according to claim 11 by folding each of the platelike bodies into a hairpin form at opposite sides of the connecting portion to butt the side wall forming portions against each other and thereafter folding the covering wall forming portion to cover outer surfaces of the two side wall forming portions therewith, preparing a pair of headers each having folded body inserting holes arranged at a spacing and fins, arranging the pair of headers as spaced apart from each other and arranging the folded bodies and the fins alternately, inserting opposite ends of the folded bodies into the respective inserting holes of the headers, fabricating flat tubes by brazing the side wall forming portions of each folded body to each other and the covering wall forming portion thereof to the outer surfaces of the side wall forming portions, and brazing the flat tubes to the headers and each fin to the flat tubes adjacent thereto simultaneously with the fabrication of the flat tubes. 